Since introduced in 1976, the dual-energy CT technology has been widely used in various fields including safety inspection, and medical care. The technology scans a subject by using two x-ray sources of different energy levels to obtain raw data at the two different energy levels. With these data, information about the subject, such as atomic number, electron density, and attenuation coefficient, can be restructured through respective image processing algorithms. The dual-energy CT technology has advantages of reconstructing not only attenuation coefficient images that can be obtained by the single-energy CT imaging technology but also atomic number and electron density information of the subject. The dual-energy CT technology is more powerful in distinguishing materials than the convention single-energy CT technology.
Currently, the dual-energy CT technology is mainly implemented in several approaches as follows. The first approach is to scan a subject twice with ray sources of different energy-levels, and radiation dose and scanning time are about twice greater than one-time scanning. Low-energy and high-energy perspective images should be registered to ensure that pixels of the same coordinates on the two images correspond to the same ray path. The second one is to utilize a ray source capable of switching a high voltage at high frequency. While the subject is passing through the view field of ray scanning, the ray source emits alternatively low-energy and high-energy rays at a very high frequency. This approach requires only one scanning process in imaging operation. The disadvantage is that pixels of the same coordinates on the low-energy and high-energy images can just correspond to adjacent ray paths. This approach is commonly adopted when an accelerator functions as ray source. The third approach is to implement dual-energy imaging with double-deck detectors specifically designed. During the scanning process, rays after penetrating a subject reach low-energy detectors first, penetrate a filter sheet, and then reach high-energy detectors. In this case, pixels on the two perspective images automatically correspond to the same ray path. Compared with the first and second approaches, this approach has smaller energy difference between high-energy and low-energy rays, and higher requirement on material recognition algorithms. Generally speaking, the previous two approaches are called true dual-energy, and the last approach is called false dual-energy.
After obtaining high-energy and low-energy projection data, the dual-energy CT technology performs dual-energy decomposition and reconstruction to obtain atomic number and distribution of electron density of the scanned subject. However, in real applications, x rays at different energy levels have different penetration capabilities, and thus the high-energy and low-energy data obtained by the dual-energy CT have signal-to-noise ratios (SNRs) significantly different from each other. This has a great impact on the final reconstruction result, and image quality is affected by noise. Meanwhile, in the reconstruction process, errors due to dual-energy decomposition and the like will degrade image quality for atomic number, and cause severe noise and artifact. Thus, it is impossible to effectively recognize structure information, thereby affecting accuracy of material recognition.